Send mail from an account on your server to an address outside your server!

And make sure you are not in authorized relayer!

(If you recently authorized on another account or checked your mail, your IP address is remembered as an authorized sender and you won't need to authorize again to send another message; In which case you might believe you have something wrong but your home IP has actually already been previously authorized by the server)

Please make sure that the service "Antirelayd " is enabled on your WHM. You can check this service from WHM --> Service Manager from Service Configuration.

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To clarify, when this service is enabled, POP before SMTP authentication will work. This means if you authenticate successfully via POP, you will not need to authenticate via SMTP for 30 minutes from that IP address.

Disabling antirelayd will force SMTP authentication for all SMTP connections, regardless of successful authentication to POP.